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September 2, 2008

Beetles Evolving as Lovers or Fighters

by Sam Savage

The evolutionary tradeoff between becoming a bigger
fighter or lover could lead to new species among dung beetle populations.

Male beetles may not transform in the blink of an
eye, but natural selection seems to have driven rapid evolution in the size of
their fighting horns - and their reproductive tools - during a time period of
just 50 years in one newly studied case.

"As horns get bigger, copulatory organs get
smaller, or vice versa," said Armin Moczek, an evolutionary biologist at
Indiana University-Bloomington. "What was not known was how frequently and
how fast this can occur in nature, and whether this can drive the evolution of
new species."

Making tradeoffs

Moczek and fellow researcher Harald Parzer examined
four geographically separate populations of a horned dung beetle species called
Onthophagus taurus. They found that the
relative investment made by each beetle population into horns and genitalia could
differ by more than three times the average investment for the overall species.

The different sizes reflect a strategy of making
investments with limited resources into either horns or genitalia. If beetles
live in low-density environments where fighting is
common, males with larger horns and smaller genitalia may find the most
success in winning mates. But if fighting is less common, having larger
genitalia at the expense of horn size may prove best.

Such tradeoffs between certain characteristics are
an "ancient and still poorly understood issue in biology," Moczek
told LiveScience. Biologists are not
surprised that secondary sexual characteristics such as horns can drive
change in the primary sexual characteristic, or the genitalia, but only two
previous studies had hinted at this in action.

Size matters?

Evolutionary biologists think that such changes in
genitalia size and shape can eventually lead to new species, when individuals
from different populations become sexually incompatible. The size of genitalia
tends to resist evolutionary change in order to preserve a species' identity,
but evolutionary pressure on the dung beetle horns has forced the changes in
genitalia as well.

"We proposed that maybe these tradeoffs are an
avenue that forces species in directions they wouldn't go with otherwise with
genitalia," Moczek said.

Individuals in most species do not choose mates
based on a "size matters" mentality toward genitalia, so the tradeoff
between the two characteristics also provides a mechanism to explain the link
between genitalia and origin
of species.

Moczek and Parzer looked at 10 other related beetle
species and found similar variety in horn and genitalia sizes, which suggests
that the same natural selection pressures continue to work after species have
split off.

"If this is all it takes to change genitalia,
it may be easier to make new species than we thought," Moczek said.

Getting around

The O. taurus
dung beetle originated in Italy, but has spread to other parts of the world
to live in far-flung populations. Humans introduced the dung beetles to
Australia in the 1960s as competitors to ward off swarms of flies that hover
over cow manure, and the beetles became so beloved there that Moczek found
himself universally welcomed down under because of his research.

"When I mentioned I was working on dung
beetles, I immediately had a beer in my hand and a place to stay," Moczek
recalled.

The beetles also showed up unannounced in the
United States during the 1960s. The separate beetle populations in the United States,
Italy, and western and eastern Australia now allow biologists to see what
evolutionary changes have occurred within the past 50 years - and perhaps
figure out where the beetles might go next.